The present invention pertains to a powder paint feeder for powder coating. More particularly, the present invention relates to a powder paint feeder for powder coating which is directly installed to a fluidized bed type powder paint tank and discharges a powder paint in a constant quantity at a pinpoint accuracy.
Powder coating for coating a powder paint without using organic solvent has been widely employed from the view point of the protection of the terrestrial environment. In case of the powder coating and, in particular, in case of electrostatic powder coating, a powder paint stored in a paint tank is sprayed onto an object by a coating gun through a powder paint feeder. Fluidized bed type powder paint tanks (hereinafter, referred to as fluidizing tanks) for applying fluidity to a powder paint by supplying air from under the powder paint have been widely used as the paint tank.
Japanese Unexamined Patent Publication No. 8-57365 discloses, as a powder paint feeder used for a fluidizing tank, an apparatus for directly taking out a powder paint, which has been fluidized in a paint tank by agitating blades, by an ejector and feeding the powder paint into a coating gun. However, the apparatus has a problem that the quantity of feed of the powder paint is changed depending upon the height of the powder level (hereinafter, referred to as the powder level) of the powder paint in the fluidizing tank because the powder paint is taken out as it is from the tank as a fluidized bed through the ejector. That is, the fall of the powder level in the paint tank reduces a powder pressure, whereby a powder volume density is also reduced. As a result, the quantity of the powder paint discharged through the ejector is also reduced. On the other hand, when a powder paint is fed into the paint tank and the powder level of the powder paint rises, the powder volume density is increased and accordingly the quantity of the powder paint fed is also increased. Thus, the feed rate of the powder paint is varied depending upon the powder level. Because of the reason mentioned above, the variation of the quantity of a discharged powder paint, which is caused by the variation of the powder level, reaches, for example, .+-.10-odd %.
Further, the apparatus has a drawback that the quantity of feed of a powder paint and the quantity of air conveyed by the ejector cannot be optionally, separately and independently set because the ejector takes out the powder paint mixed with the air fluidized in the fluidizing tank.
Whereas, Japanese Unexamined Patent Publication No. 8-47656 discloses an apparatus for feeding a powder paint into a coating gun in a constant quantity to prevent the variation of the quantity of feed of the powder paint caused by the variation of the powder level of it. The apparatus is arranged such that the powder paint intermittently fed from a fluidizing tank is stored once in a sub-tank, fluidized air is released from the sub-tank using a vibrator or the like, and the powder paint is fed into a coating gun by a screw feeder disposed at a lower portion of the sub-tank. In the constant quantity feeding apparatus disclosed in the publication, a constant quantity feeding property is improved by adjusting the quantity of a discharged powder paint by the rotation of a screw.
However, the constant quantity feeding apparatus disclosed in the publication has a drawback that since the apparatus is made large in size and complicated in construction by the provision of the sub-tank and the vibrator, the cost of the apparatus is increased as compared with the cost of the above ejector type powder paint feeder as well as a long time is necessary to disassemble and wash the apparatus, although the apparatus can reduce the variation of the quantity of a discharged power paint which is caused by a powder level to about .+-.2% by the installation of the sub-tank.
Further, there is also proposed a constant quantity powder paint feeder the arrangement of which is simplified by omitting a sub-tank or a hopper unit and directly disposing a screw feeder to a fluidizing tank to improve a disassembling property and a washing property and to reduce a cost.
However, the constant quantity powder paint feeder has a problem that since the feeder is not provided with a sub-tank, fluidized air cannot be sufficiently released, a powder volume density is varied by being affected by a powder level, and a sufficiently high constant quantity feeding property cannot be obtained; that is, a quantity of feed is varied about .+-.7% based on the quantity of a discharged powder paint by the variation of the powder level. Further, the feeder also has a drawback that when a highly fluid powder paint is used, countermeasures against a flushing phenomenon must be taken by employing a mechanical method such as the installation of a straightening vane, or the like.